1. Field of the Invention
The present invention relates to a method of and an apparatus for processing a workpiece in a plasma, e.g., etching a wafer in a plasma, or depositing a layer on a wafer in a plasma by way of chemical vapor deposition (CVD).
2. Description of the Relevant Art
There are known plasma processing apparatus which generate a plasma to activate a reaction gas for etching a wafer or depositing a layer on a wafer by way of CVD. One such plasma processing apparatus is known as an electron cyclotron resonance (ECR) plasma CVD system that produces a high-density plasma in a high vacuum with electrons moving in cyclotron orbit in the presence of a magnetic field and a microwave for etching or otherwise processing workpieces highly accurately and efficiently. The ECR plasma CVD system generally has a reaction chamber disposed in a casing connected to a waveguide coupled to a microwave oscillator, and a coil disposed around the reaction chamber for generating a magnetic field in the reaction chamber.
Conventional plasma processing apparatus are disclosed in U.S. Pat. Nos. 4,298,419; 4,330,384; 4,430,138; 4,462,863; 4,559,100; and 4,705,595, or example.
FIGS. 1 (a) and 1 (b) of the accompanying drawings show the manner in which a magnetic field affects a plasma generated in a conventional ECR plasma CVD system. As shown in FIG. 1 (a), the plasma is constricted into a high-density state within a coil 11 because the plasma tends to avoid a region where the flux density is high. Outside of the coil 11, however, the flux density is lower as shown in FIG. 1 (b) . Consequently, the plasma as it reaches a wafer W on a table 13 outside of the coil 11 is relatively uneven while the wafer W is being etched, causing an undercut in a peripheral edge of the wafer W or etching the surface of the wafer W at different etching rates.
Another known ECR plasma CVD system, shown in FIGS. 2 (a) and 2 (b) of the accompanying drawings, has upper and lower coils 11, 12 arranged successively along the axial direction of a reaction chamber for further constricting a plasma generated in the reaction chamber. The ECR plasma CVD system shown in FIGS. 2(a) and 2(b) is however unable to fully prevent the plasma from being uneven at the wafer W.